1. Field of the Invention
The present invention relates to an image position detecting device suitable for use with an auto-focusing device in a camera. More particularly, the present invention relates to an image position detecting device which detects the relative positions of the image of an object on a pair of image sensors.
2. Description of the Related Art
Generally, conventional auto-focusing optical instrument have utilized one of two systems, either an active system for measuring the distance from the optical instrument to an object using infrared rays, or a passive system for measuring the distance by using light reflected from an object. To detect this distance, and the related distance from an in-focus position, the passive system generally projects an image of an object on a pair of image sensors, each sensor consisting of a number of photosensor elements. The passive system then electronically detects the relative positions of the image on the image sensors. A typical example of a conventional passive image position detecting device that utilizes these concepts will now be described.
FIG. 8 shows a conventional system for distance detection based on the image position. As shown, a pair of small lenses 2L and 2R (L indicating left and R indicating right) for projecting an image of an object 1 are displaced a distance "d" from the object. The lenses 2L and 2R are disposed at different positions separated by the length "b". Images 4L and 4R of the object 1 are focused at positions PL and PR on the image sensors 3L and 3R disposed near the lenses 2L and 2R. If the object 1 is located at an infinitely distant point, the image of the object 1 is focused on reference positions PO, where two parallel optical paths LO interest the image sensors 3L and 3R. Assuming that the displacement of the positions PL and PR from the reference points PO are xL and xR, respectively, the distance "d" from the optical instrument to the object 1 may be expressed as: EQU d=bf/(xL+xR).
In the above equation, "b" and "f" are known parameters determined by the optical instrument. Therefore, as seen from the equation, the distance "d" can be determined by using the relative positions of the images 4L and 4R on the image sensors 3L and 3R. More specifically, the distance "d" is a function of the sum of the displacements xL and xR of those images from the reference positions PO.
FIG. 9 shows a diagram explaining how the displacements xL and xR are summed. Groups of image data 5L and 5R, obtained by the pair of image sensors 3L and 3R, are illustrated in the upper portion of FIG. 9 Each group of image data consists of a set of pixels equal in number to the photosensors in each image sensor. Each pixel of image data is a digital value representing the intensity of light received by the photosensor.
As represented two dimensionally in FIG. 9, image data group 5L consists of (n+1) number of image data elements L.sub.o to L.sub.n. Each image data element may contain one or more pixels. Similarly, the image data group 5R consists of (n+1) number of image data elements R.sub.o to R.sub.n. The image data groups each contain the image patterns 4L and 4R shown in FIG. 8.
In order to detect the relative positions of images 4L and 4R, imaginary windows 6, hatched as shown in FIG. 8, are used. The partial data groups defined by the windows 6 are selected from the image data 5L and 5R shown in FIG. 9. The correlation between the paired partial data groups is checked. Specifically, it is determined whether the two partial data groups are coincident with each other. After the determination is made, the two partial data groups are changed and the correlation is checked again. Several possible combinations of the partial data groups are illustrated under the right and left image data groups 5R and 5L shown in FIG. 9.
The partial data group defined by each window 6 consists of (m+1) number of image data elements (where n&gt;m). For the first combination C of the partial image data groups, the correlation between the two partial data groups is checked (the first group consisting of image data elements R.sub.n to R.sub.n-m from the left end of the right image data group 5R). As is readily seen, when the object is located at an infinitely distant point, the combination C.sub.o of the image data groups will exhibit the highest correlation.
For the combination C.sub.1, the right partial data group is displaced by one element of image data. As shown in FIG. 9, the combination C.sub.1 includes a right image data group from R.sub.n-1 through R.sub.n-m-1. Similarly, the left partial data group associated with combination C.sub.2 is displaced one element of image data. The remainder of the combinations are alternately displaced in a similar fashion.
The combination of the left and right partial data groups is generally expressed by C.sub.i, where "i"=0 to 2n-2m. Further, assume that of those combinations, the k-the combination ("C.sub.k ") exhibits the highest correlation. As is readily seen, the value k of the combination C.sub.k may be used as an index which is proportional to the sum xL+xR of the displacements. Therefore, the constant of proportionality between the index value k and the sum xL+xR is equal to the linear density of the array of the photosensors which comprises the image sensor.
The operation of a conventional distance detecting device which uses the positions of the images on the image sensor was previously described. In the automatic focusing operation of a camera, for example, it is common practice that the index "i" is used directly, without calculating the distance "d". While only the principle of distance detection has been discussed, it should be understood that image position detection may be used for other purposes, such as a part of a focusing system in an optical instrument.
In a conventional distance detection operation, the size of the windows for detecting the relative positions of an object defines the width of the field of detection and the angle of the field of detection. Accordingly, accurate detection requires that neither the field angle be too wide nor too narrow. Therefore, the window size is empirically optimized to obtain the highest detection accuracy. However, when the optical parameters of a photographic lens, for example, are changed, the optimized condition may be lost, and the detection accuracy may be accordingly impaired.
In conventional cameras, interchangeable lenses and zoom lenses are frequently used. When such lenses are used, the field angle may be greatly changed. Frequently, the field angle is optimized for a standard lens or zoom. When another lens is used or the camera is used at a different zoom, the object is therefore frequently detected using an improper field angle. This phenomenon will now be described in detail with reference to FIGS. 6 and 7.
FIG. 6 shows a field angle for image position detection. A field angle ".alpha." is given by EQU .alpha.=2 arctan (w/2f),
where "f" is the focal distance of the small lens 2, and "w" is the width of the window 6 used for detecting a position of an image 4, which is imaged on the image sensor 3 through the lens 2.
FIG. 7 shows a photographic angle ".beta." of a film photographing system. A photographic angle ".beta." is expressed by EQU .beta.=2 arctan (h/2fz),
where "fz" is the focal length of a photographic lens 7, and "h" is the width of a film 8 on which the image 4 and background are imaged through the lens 7.
Generally, the detection field angle .alpha. is set to be smaller than the photographic angel ".beta.". A field ratio Q (=.alpha./.beta.) indicates the portion of the photographic angle used for detecting the position of an image. For the standard lens, an optimum value, which may generally be equal to or less than "1", is empirically selected. When the standard lens is replaced by a wide angle lens, however, the photographic angle ".beta." becomes large, and hence the field ratio Q becomes excessively small. When the standard lens is replaced by a telephoto lens, the photographic angle becomes small and the field ratio becomes excessively large. In both cases, the image position detection accuracy is degraded.